1. Field of the Invention
The present invention relates to a failure detection apparatus for an internal combustion engine, and more particularly, to techniques enabling reliable detection of abnormality in an air flow sensor.
2. Description of the Related Art
Recently, in order to prevent harmful exhaust gas from being emitted from an engine mounted on a motor vehicle, various control means are used to improve exhaust gas characteristics. These control means operate based on information supplied from various sensors and the like, to improve the exhaust gas characteristics.
If any of the sensors etc. fails, however, the exhaust gas characteristics may possibly be deteriorated, and accordingly, there has been a demand for reliable detection of failure of such sensors etc. Recently, vehicles equipped with an on-board diagnostic system (OBD etc.) have been developed and put to practical use, with a view to further improving the exhaust gas characteristics.
Information from the various sensors etc., especially, information from an air flow sensor (AFS) is used for many purposes including the control of an after-treatment device, EGR or exhaust gas recirculation, etc., and failure of the air flow sensor greatly affects the exhaust gas characteristics. Accordingly, diagnosis of the air flow sensor is of especial importance.
Thus, an apparatus has been proposed in which, when the rotation speed of the engine is lower than or equal to a predetermined value and at the same time an intake air quantity detected by the air flow sensor is higher than or equal to a predetermined value, for example, the air flow sensor is judged to be abnormal (see Japanese Patent Application Publication No. H10-018897, for example).
In such diagnosis of the air flow sensor, failure of the sensor is usually determined by comparing a reference value set in advance based on operating conditions, such as engine speed, fuel injection quantity, engine torque, throttle opening (throttle valve opening), manifold air pressure, etc., with an output value from the air flow sensor.
However, in the case where the exhaust flow rate (exhaust pressure) is adjusted by means of the variable nozzles/vanes of a variable nozzles/vanes-equipped supercharger (VG turbocharger) or where EGR gas is introduced into the intake system while the exhaust flow rate is adjusted by the variable nozzles/vanes, the quantity of fresh air varies depending on the vane opening of the variable nozzles/vanes or the opening of the EGR valve, giving rise to a problem that failure of the air flow sensor cannot be determined with accuracy.
To solve the problem, the diagnosis of the air flow sensor may be suspended while the vane opening or the EGR valve opening varies, for example. In the apparatus disclosed in the aforementioned publication, the diagnosis of the air flow sensor as to abnormality is prohibited when the engine is in a cold state in which the engine is supplied with bypass air.
Such suspension of the diagnosis is, however, not desirable because it leads to a substantial reduction in the period for diagnosing the air flow sensor.